Reversibly reducible metal complexes as electron transporting materials for oleds

ABSTRACT

The invention is reversibly reducible metal complexes and materials and an organic light emitting device, having an anode; a cathode; and at least one organic layer disposed between the anode and the cathode, made with the complexes of the invention. The reversibly reducible metal complexes are complexes a redox active metal center and at least one ligand; wherein; following a reduction of the complex, adding 1 extra electron to the complex, the extra electron is localized on the metal center. The complexes may function as an ETL or a host material.

This application is a continuation of U.S. application Ser. No.11/032,941, filed Jan. 10, 2005, which is related to U.S. applicationSer. No. 10/880,384, filed Jun. 28, 2004, entitled Luminescent Compoundswith Carbene Ligands, each of which is incorporated by reference in itsentirety.

The claimed invention was made by, on behalf of, and/or in connectionwith one or more of the following parties to a joint universitycorporation research agreement: Princeton University, The University ofSouthern California, The University of Michigan and Universal DisplayCorporation. The agreement was in effect on and before the date theclaimed invention was made, and the claimed invention was made as aresult of activities undertaken within the scope of the agreement.

FIELD OF THE INVENTION

The present invention relates to organic light emitting devices (OLEDs),and, more specifically, to electron transport and host materials forsuch devices. In particular, the present invention relates to OLEDShaving one or more layers comprising a redox active metal complex, i.e.,a reversibly reducible metal complex.

BACKGROUND

Opto-electronic devices that make use of organic materials are becomingincreasingly desirable for a number of reasons. Many of the materialsused to make such devices are relatively inexpensive, so organicopto-electronic devices have the potential for cost advantages overinorganic devices. In addition, the inherent properties of organicmaterials, such as their flexibility, may make them well suited forparticular applications such as fabrication on a flexible substrate.Examples of organic opto-electronic devices include organic lightemitting devices (OLEDs), organic phototransistors, organic photovoltaiccells, and organic photodetectors. For OLEDs, the organic materials mayhave performance advantages over conventional materials. For example,the wavelength at which an organic emissive layer emits light maygenerally be readily tuned with appropriate dopants.

As used herein, the term “organic” includes polymeric materials as wellas small molecule organic materials that may be used to fabricateorganic opto-electronic devices. “Small molecule” refers to any organicmaterial that is not a polymer, and “small molecules” may actually bequite large. Small molecules may include repeat units in somecircumstances. For example, using a long chain alkyl group as asubstituent does not remove a molecule from the “small molecule” class.Small molecules may also be incorporated into polymers, for example as apendent group on a polymer backbone or as a part of the backbone. Smallmolecules may also serve as the core moiety of a dendrimer, whichconsists of a series of chemical shells built on the core moiety. Thecore moiety of a dendrimer may be a fluorescent or phosphorescent smallmolecule emitter. A dendrimer may be a “small molecule,” and it isbelieved that all dendrimers currently used in the field of OLEDs aresmall molecules. In general, a small molecule has a well-definedchemical formula with a single molecular weight, whereas a polymer has achemical formula and a molecular weight that may vary from molecule tomolecule. As used herein, “organic” includes metal complexes ofhydrocarbyl and heteroatom-substituted hydrocarbyl ligands.

OLEDs make use of thin organic films that emit light when voltage isapplied across the device. OLEDs are becoming an increasinglyinteresting technology for use in applications such as flat paneldisplays, illumination, and backlighting. Several OLED materials andconfigurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and5,707,745, which are incorporated herein by reference in their entirety.

OLED devices are generally (but not always) intended to emit lightthrough at least one of the electrodes, and one or more transparentelectrodes may be useful in an organic opto-electronic devices. Forexample, a transparent electrode material, such as indium tin oxide(ITO), may be used as the bottom electrode. A transparent top electrode,such as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745, which areincorporated by reference in their entireties, may also be used. For adevice intended to emit light only through the bottom electrode, the topelectrode does not need to be transparent, and may comprise a thick andreflective metal layer having a high electrical conductivity. Similarly,for a device intended to emit light only through the top electrode, thebottom electrode may be opaque and/or reflective. Where an electrodedoes not need to be transparent, using a thicker layer may providebetter conductivity, and using a reflective electrode may increase theamount of light emitted through the other electrode, by reflecting lightback towards the transparent electrode. Fully transparent devices mayalso be fabricated, where both electrodes are transparent. Side emittingOLEDs may also be fabricated, and one or both electrodes may be opaqueor reflective in such devices.

As used herein, “top” means furthest away from the substrate, while“bottom” means closest to the substrate. For example, for a devicehaving two electrodes, the bottom electrode is the electrode closest tothe substrate, and is generally the first electrode fabricated. Thebottom electrode has two surfaces, a bottom surface closest to thesubstrate, and a top surface further away from the substrate. Where afirst layer is described as “disposed over” a second layer, the firstlayer is disposed further away from substrate. There may be other layersbetween the first and second layer, unless it is specified that thefirst layer is “in physical contact with” the second layer. For example,a cathode may be described as “disposed over” an anode, even thoughthere are various organic layers in between.

As used herein, “solution processible” means capable of being dissolved,dispersed, or transported in and/or deposited from a liquid medium,either in solution or suspension form.

As used herein, and as would be generally understood by one skilled inthe art, a first “Highest Occupied Molecular Orbital” (HOMO) or “LowestUnoccupied Molecular Orbital” (LUMO) energy level is “greater than” or“higher than” a second HOMO or LUMO energy level if the first energylevel is closer to the vacuum energy level. Since ionization potentials(EP) are measured as a negative energy relative to a vacuum level, ahigher HOMO energy level corresponds to an IP having a smaller absolutevalue (an IP that is less negative). Similarly, a higher LUMO energylevel corresponds to an electron affinity (EA) having a smaller absolutevalue (an EA that is less negative). On a conventional energy leveldiagram, with the vacuum level at the top, the LUMO energy level of amaterial is higher than the HOMO energy level of the same material. A“higher” HOMO or LUMO energy level appears closer to the top of such adiagram than a “lower” HOMO or LUMO energy level.

OLEDs typically comprise at least one organic layer, sandwiched betweenthe top and bottom electrodes. For example, an OLED may comprise anorganic electron-transporting layer (“ETL”), an organic emissive layer,and/or an organic hole-transporting (“HTL”) or hole-injection layer(“HIL”). Prior art OLEDs typically comprise a metal complex, such asAlq₃ or Balq as an ETL material. Such metal complexes comprise ligandscapable of accepting an electron, However, the choice of such compoundsis limited.

Therefore, a new class of ETL materials is desirable. The presentinvention provides such a new class of ETL materials.

SUMMARY OF THE INVENTION

The invention is directed to reversibly reducible metal complexes and toETLs and organic light emitting devices, i.e., OLEDs, incorporating thereversibly reducible metal complex of the invention. OLEDs,incorporating a reversibly reducible metal complex of the invention,comprise an anode; a cathode; and at least one organic layer disposedbetween the anode and the cathode, where the organic layer comprises areversibly reducible metal complex of a redox active metal center and atleast one ligand, where, in a reduced state, an extra electron islocalized on the metal center. The redox active metal complex, i.e., thereversibly reducible metal complex, may function as an ETL and/or as ahost for an emissive material. Preferred reducible metal complexes inaccordance with the invention are of Formula I

and Formula II

where M is a redox active transition metal, preferably a redox active d0transition metal; E¹ and E³ are independently selected from nitrogen,oxygen, and sulfur mono-anionic donors; E² is a C, N, P, or Si neutraldonor; (X-Y) is selected from a photoactive ligand or an ancillaryligand; m is a value from 1 to the maximum number of ligands that may beattached to metal M; and m+n is the maximum number of ligands that maybe attached to metal M, such that the overall complex is neutral.

Redox active metal complexes in accordance with the invention include,but are not limited to, reversibly reducible metal complexes of formula:

where E¹ and E³ are either/or nitrogen, oxygen, or sulfur mono anionicdonors, and E² is a neutral donor, such as C, N, P, Si, etc. Inaddition, bulky alkyl and aryl substituents may be used to stabilize thecomplexes, i.e., R1-R6 may be hydrogen or hydrocarbon groups.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an organic light emitting device having separate electrontransport, hole transport, and emissive layers, as well as other layers.

FIG. 2 shows an inverted organic light emitting device that does nothave a separate electron transport layer.

DETAILED DESCRIPTION

Generally, an OLED comprises at least one organic layer disposed betweenand electrically connected to an anode and a cathode. When a current isapplied, the anode injects holes and the cathode injects electrons intothe organic layer(s). The injected holes and electrons each migratetoward the oppositely charged electrode. When an electron and holelocalize on the same molecule, an “exciton,” which is a localizedelectron-hole pair having an excited energy state, is formed. Light isemitted when the exciton relaxes via a photoemissive mechanism. In somecases, the exciton may be localized on an excimer or an exciplex.Non-radiative mechanisms, such as thermal relaxation, may also occur,but are generally considered undesirable.

The initial OLEDs used emissive molecules that emitted light from theirsinglet states (“fluorescence”), as disclosed, for example, in U.S. Pat.No. 4,769,292, which is incorporated by reference in its entirety.Fluorescent emission generally occurs in a time frame of less than 10nanoseconds.

More recently, OLEDs having emissive materials that emit light fromtriplet states (“phosphorescence”) have been demonstrated. Baldo et al.,“Highly Efficient Phosphorescent Emission from OrganicElectroluminescent Devices,” Nature, vol. 395, 151-154, 1998;(“Baldo-I”) and Baldo et al., “Very high-efficiency green organiclight-emitting devices based on electrophosphorescence,” Appl. Phys.Lett., vol. 75, No. 3, 4-6 (1999) (“Baldo-II”), which are incorporatedby reference in their entireties. Phosphorescence may be referred to asa “forbidden” transition because the transition requires a change inspin states, and quantum mechanics indicates that such a transition isnot favored. As a result, phosphorescence generally occurs in a timeframe exceeding at least 10 nanoseconds, and typically greater than 100nanoseconds. If the natural radiative lifetime of phosphorescence is toolong, triplets may decay by a non-radiative mechanism, such that nolight is emitted. Organic phosphorescence is also often observed inmolecules containing heteroatoms with unshared pairs of electrons atvery low temperatures. 2,2′-bipyridine is such a molecule. Non-radiativedecay mechanisms are typically temperature dependent, such that anorganic material that exhibits phosphorescence at liquid nitrogentemperatures typically does not exhibit phosphorescence at roomtemperature. But, as demonstrated by Baldo, this problem may beaddressed by selecting phosphorescent compounds that do phosphoresce atroom temperature. Representative emissive layers include doped orun-doped phosphorescent organometallic materials such as disclosed inU.S. Pat. Nos. 6,303,238 and 6,310,360; U.S. Patent ApplicationPublication Nos. 2002-0034656; 2002-0182441; 2003-0072964; andWO-02/074015.

Generally, the excitons in an OLED are believed to be created in a ratioof about 3:1, i.e., approximately 75% triplets and 25% singlets. See,Adachi et al., “Nearly 100% Internal Phosphorescent Efficiency In AnOrganic Light Emitting Device,” J. Appl. Phys., 90, 5048 (2001), whichis incorporated by reference in its entirety. In many cases, singletexcitons may readily transfer their energy to triplet excited states via“intersystem crossing,” whereas triplet excitons may not readilytransfer their energy to singlet excited states. As a result, 100%internal quantum efficiency is theoretically possible withphosphorescent OLEDs. In a fluorescent device, the energy of tripletexcitons is generally lost to radiationless decay processes that heat-upthe device, resulting in much lower internal quantum efficiencies. OLEDsutilizing phosphorescent materials that emit from triplet excited statesare disclosed, for example, in U.S. Pat. No. 6,303,238, which isincorporated by reference in its entirety.

Phosphorescence may be preceded by a transition from a triplet excitedstate to an intermediate non-triplet state from which the emissive decayoccurs. For example, organic molecules coordinated to lanthanideelements often phosphoresce from excited states localized on thelanthanide metal. However, such materials do not phosphoresce directlyfrom a triplet excited state but instead emit from an atomic excitedstate centered on the lanthanide metal ion. The europium diketonatecomplexes illustrate one group of these types of species.

Phosphorescence from triplets can be enhanced over fluorescence byconfining, preferably through bonding, the organic molecule in closeproximity to an atom of high atomic number. This phenomenon, called theheavy atom effect, is created by a mechanism known as spin-orbitcoupling. Such a phosphorescent transition may be observed from anexcited metal-to-ligand charge transfer (MLCT) state of anorganometallic molecule such as tris(2-phenylpyridine)iridium(III).

As used herein, the term “triplet energy” refers to an energycorresponding to the highest energy feature discernable in thephosphorescence spectrum of a given material. The highest energy featureis not necessarily the peak having the greatest intensity in thephosphorescence spectrum, and could, for example, be a local maximum ofa clear shoulder on the high energy side of such a peak.

The term “organometallic” as used herein is as generally understood byone of ordinary skill in the art and as given, for example, in“Inorganic Chemistry” (2nd Edition) by Gary L. Miessler and Donald A.Tarr, Prentice Hall (1998). Thus, the term organometallic refers tocompounds which have an organic group bonded to a metal through acarbon-metal bond. This class does not include per se coordinationcompounds, which are substances having only donor bonds fromheteroatoms, such as metal complexes of amines, halides, pseudohalides(CN, etc.), and the like. In practice organometallic compounds generallycomprise, in addition to one or more carbon-metal bonds to an organicspecies, one or more donor bonds from a heteroatom. The carbon-metalbond to an organic species refers to a direct bond between a metal and acarbon atom of an organic group, such as phenyl, alkyl, alkenyl, etc.,but does not refer to a metal bond to an “inorganic carbon,” such as thecarbon of CN or CO.

FIG. 1 shows an organic light emitting device 100. The figures are notnecessarily drawn to scale. Device 100 may include a substrate 110, ananode 115, a hole injection layer 120, a hole transport layer 125, anelectron blocking layer 130, an emissive layer 135, a hole blockinglayer 140, an electron transport layer 145, an electron injection layer150, a protective layer 155, and a cathode 160. Cathode 160 is acompound cathode having a first conductive layer 162 and a secondconductive layer 164. Device 100 may be fabricated by depositing thelayers described, in order.

Substrate 110 may be any suitable substrate that provides desiredstructural properties. Substrate 110 may be flexible or rigid. Substrate110 may be transparent, translucent or opaque. Plastic and glass areexamples of preferred rigid substrate materials. Plastic and metal foilsare examples of preferred flexible substrate materials. Substrate 110may be a semiconductor material in order to facilitate the fabricationof circuitry. For example, substrate 110 may be a silicon wafer uponwhich circuits are fabricated, capable of controlling OLEDs subsequentlydeposited on the substrate. Other substrates may be used. The materialand thickness of substrate 110 may be chosen to obtain desiredstructural and optical properties.

Anode 115 may be any suitable anode that is sufficiently conductive totransport holes to the organic layers. The material of anode 115preferably has a work function higher than about 4 eV (a “high workfunction material”). Preferred anode materials include conductive metaloxides, such as indium tin oxide (ITO) and indium zinc oxide (IZO),aluminum zinc oxide (AlZnO), and metals. Anode 115 (and substrate 110)may be sufficiently transparent to create a bottom-emitting device. Apreferred transparent substrate and anode combination is commerciallyavailable ITO (anode) deposited on glass or plastic (substrate). Aflexible and transparent substrate-anode combination is disclosed inU.S. Pat. Nos. 5,844,363 and 6,602,540 B2, which are incorporated byreference in their entireties. Anode 115 may be opaque and/orreflective. A reflective anode 115 may be preferred for sometop-emitting devices, to increase the amount of light emitted from thetop of the device. The material and thickness of anode 115 may be chosento obtain desired conductive and optical properties. Where anode 115 istransparent, there may be a range of thickness for a particular materialthat is thick enough to provide the desired conductivity, yet thinenough to provide the desired degree of transparency. Other anodematerials and structures may be used.

Hole transport layer 125 may include a material capable of transportingholes. Hole transport layer 130 may be intrinsic (undoped), or doped.Doping may be used to enhance conductivity. α-NPD and TPD are examplesof intrinsic hole transport layers. An example of a p-doped holetransport layer is m-MTDATA doped with F₄-TCNQ at a molar ratio of 50:1,as disclosed in United States Patent Application Publication No.2003-0230980 to Forrest et al., which is incorporated by reference inits entirety. Other hole transport layers may be used.

Emissive layer 135 may include an organic material capable of emittinglight when a current is passed between anode 115 and cathode 160.Preferably, emissive layer 135 contains a phosphorescent emissivematerial, although fluorescent emissive materials may also be used.Phosphorescent materials are preferred because of the higher luminescentefficiencies associated with such materials. Emissive layer 135 may alsocomprise a host material capable of transporting electrons and/or holes,doped with an emissive material that may trap electrons, holes, and/orexcitons, such that excitons relax from the emissive material via aphotoemissive mechanism. Emissive layer 135 may comprise a singlematerial that combines transport and emissive properties. Whether theemissive material is a dopant or a major constituent, emissive layer 135may comprise other materials, such as dopants that tune the emission ofthe emissive material. Emissive layer 135 may include a plurality ofemissive materials capable of, in combination, emitting a desiredspectrum of light. Examples of phosphorescent emissive materials includeIr(ppy)₃. Examples of fluorescent emissive materials include DCM andDMQA. Examples of host materials include reducible complexes of redoxactive metals and ligands, as discussed below, as well as Alq₃, CBP andmCP. Examples of emissive and host materials are disclosed in U.S. Pat.No. 6,303,238 to Thompson et al., which is incorporated by reference inits entirety. An emissive material may be included in emissive layer 135in a number of ways. For example, an emissive small molecule may beincorporated into a polymer. This may be accomplished by several ways:by doping the small molecule into the polymer either as a separate anddistinct molecular species; or by incorporating the small molecule intothe backbone of the polymer, so as to form a co-polymer; or by bondingthe small molecule as a pendant group on the polymer. Other emissivelayer materials and structures may be used. For example, a smallmolecule emissive material may be present as the core of a dendrimer.

Many useful emissive materials include one or more ligands bound to ametal center. A ligand may be referred to as “photoactive” if itcontributes directly to the photoactive properties of an organometallicemissive material. A “photoactive” ligand may provide, in conjunctionwith a metal, the energy levels from which and to which an electronmoves when a photon is emitted. Other ligands may be referred to as“ancillary.” Ancillary ligands may modify the photoactive properties ofthe molecule, for example by shifting the energy levels of a photoactiveligand, but ancillary ligands do not directly provide the energy levelsinvolved in light emission. A ligand that is photoactive in one moleculemay be ancillary in another. These definitions of photoactive andancillary are intended as non-limiting theories.

Electron transport layer 145 may include a material capable oftransporting electrons. Electron transport layer 145 may be intrinsic(undoped), or doped. Doping may be used to enhance conductivity. Alq₃ isan example of an intrinsic electron transport layer. An example of ann-doped electron transport layer is BPhen doped with Li at a molar ratioof 1:1, as disclosed in United States Patent Application Publication No.2003-0230980 to Forrest et al., which is incorporated by reference inits entirety. Other electron transport layers known in the art may beused. Preferably, ETL 145 comprises a reversibly reducible complex of aredox active metal and at least one ligand, as discussed below, aseither the electron transport material or as a host with at least onedopant.

The charge carrying component of the electron transport layer may beselected such that electrons can be efficiently injected from thecathode into the LUMO (Lowest Unoccupied Molecular Orbital) energy levelof the electron transport layer. The “charge carrying component” is thematerial responsible for the LUMO energy level that actually transportselectrons. This component may be the base material, or it may be adopant. The LUMO energy level of an organic material may be generallycharacterized by the electron affinity of that material and the relativeelectron injection efficiency of a cathode may be generallycharacterized in terms of the work function of the cathode material.This means that the preferred properties of an electron transport layerand the adjacent cathode may be specified in terms of the electronaffinity of the charge carrying component of the ETL and the workfunction of the cathode material. In particular, so as to achieve highelectron injection efficiency, the work function of the cathode materialis preferably not greater than the electron affinity of the chargecarrying component of the electron transport layer by more than about0.75 eV, more preferably, by not more than about 0.5 eV. Similarconsiderations apply to any layer into which electrons are beinginjected.

Cathode 160 may be any suitable material or combination of materialsknown to the art, such that cathode 160 is capable of conductingelectrons and injecting them into the organic layers of device 100.Cathode 160 may be transparent or opaque, and may be reflective. Metalsand metal oxides are examples of suitable cathode materials. Cathode 160may be a single layer, or may have a compound structure. FIG. 1 shows acompound cathode 160 having a thin metal layer 162 and a thickerconductive metal oxide layer 164. In a compound cathode, preferredmaterials for the thicker layer 164 include ITO, IZO, and othermaterials known to the art. U.S. Pat. Nos. 5,703,436, 5,707,745,6,548,956 B2, and 6,576,134 B2, which are incorporated by reference intheir entireties, disclose examples of cathodes including compoundcathodes having a thin layer of metal such as Mg:Ag with an overlyingtransparent, electrically-conductive, sputter-deposited ITO layer. Thepart of cathode 160 that is in contact with the underlying organiclayer, whether it is a single layer cathode 160, the thin metal layer162 of a compound cathode, or some other part, is preferably made of amaterial having a work function lower than about 4 eV (a “low workfunction material”). Other cathode materials and structures may be used.

Blocking layers may be used to reduce the number of charge carriers(electrons or holes) and/or excitons that leave the emissive layer. Anelectron blocking layer 130 may be disposed between emissive layer 135and the hole transport layer 125, to block electrons from leavingemissive layer 135 in the direction of hole transport layer 125.Similarly, a hole blocking layer 140 may be disposed between emissivelayer 135 and electron transport layer 145, to block holes from leavingemissive layer 135 in the direction of electron transport layer 145.Blocking layers may also be used to block excitons from diffusing out ofthe emissive layer. The theory and use of blocking layers is describedin more detail in U.S. Pat. No. 6,097,147 and United States PatentApplication Publication No. 2003-0230980 to Forrest et al., which areincorporated by reference in their entireties.

As used herein, as would be understood by one skilled in the art, theterm “blocking layer” means that the layer provides a barrier thatsignificantly inhibits transport of charge carriers and/or excitonsthrough the device, without suggesting that the layer necessarilycompletely blocks the charge carriers and/or excitons. The presence ofsuch a blocking layer in a device may result in substantially higherefficiencies as compared to a similar device lacking a blocking layer.Also, a blocking layer may be used to confine emission to a desiredregion of an OLED.

Generally, injection layers comprise a material that may improve theinjection of charge carriers from one layer, such as an electrode or anorganic layer, into an adjacent organic layer. Injection layers may alsoperform a charge transport function. In device 100, hole injection layer120 may be any layer that improves the injection of holes from anode 115into hole transport layer 125. CuPc is an example of a material that maybe used as a hole injection layer from an ITO anode 115, and otheranodes. In device 100, electron injection layer 150 may be any layerthat improves the injection of electrons into electron transport layer145. LiF/Al is an example of a material that may be used as an electroninjection layer into an electron transport layer from an adjacent layer.Other materials or combinations of materials may be used for injectionlayers. Depending upon the configuration of a particular device,injection layers may be disposed at locations different than those shownin device 100. More examples of injection layers are provided in U.S.patent application Ser. No. 09/931,948 to Lu et al., which isincorporated by reference in its entirety. A hole injection layer maycomprise a solution deposited material, such as a spin-coated polymer,e.g., PEDOT:PSS, or it may be a vapor deposited small molecule material,e.g., CuPc or MTDATA.

A hole injection layer (HIL) may planarize or wet the anode surface soas to provide efficient hole injection from the anode into the holeinjecting material. A hole injection layer may also have a chargecarrying component having HOMO (Highest Occupied Molecular Orbital)energy levels that favorably match up, as defined by theirherein-described relative ionization potential (IP) energies, with theadjacent anode layer on one side of the HIL and the hole transportinglayer on the opposite side of the HIL. The “charge carrying component”is the material responsible for the HOMO energy level that actuallytransports holes. This component may be the base material of the HIL, orit may be a dopant. Using a doped HIL allows the dopant to be selectedfor its electrical properties, and the host to be selected formorphological properties such as wetting, flexibility, toughness, etc.Preferred properties for the HIL material are such that holes can beefficiently injected from the anode into the HIL material. Inparticular, the charge carrying component of the HIL preferably has anIP not more than about 0.7 eV greater that the IP of the anode material.More preferably, the charge carrying component has an IP not more thanabout 0.5 eV greater than the anode material. Similar considerationsapply to any layer into which holes are being injected. HIL materialsare further distinguished from conventional hole transporting materialsthat are typically used in the hole transporting layer of an OLED inthat such HIL materials may have a hole conductivity that issubstantially less than the hole conductivity of conventional holetransporting materials. The thickness of the HIL of the presentinvention may be thick enough to help planarize or wet the surface ofthe anode layer. For example, an HIL thickness of as little as 10 nm maybe acceptable for a very smooth anode surface. However, since anodesurfaces tend to be very rough, a thickness for the HIL of up to 50 nmmay be desired in some cases.

A protective layer may be used to protect underlying layers duringsubsequent fabrication processes. For example, the processes used tofabricate metal or metal oxide top electrodes may damage organic layers,and a protective layer may be used to reduce or eliminate such damage.In device 100, protective layer 155 may reduce damage to underlyingorganic layers during the fabrication of cathode 160. Preferably, aprotective layer has a high carrier mobility for the type of carrierthat it transports (electrons in device 100), such that it does notsignificantly increase the operating voltage of device 100. CuPc, BCP,and various metal phthalocyanines are examples of materials that may beused in protective layers. Other materials or combinations of materialsmay be used. The thickness of protective layer 155 is preferably thickenough that there is little or no damage to underlying layers due tofabrication processes that occur after organic protective layer 160 isdeposited, yet not so thick as to significantly increase the operatingvoltage of device 100. Protective layer 155 may be doped to increase itsconductivity. For example, a CuPc or BCP protective layer 160 may bedoped with Li. A more detailed description of protective layers may befound in U.S. patent application Ser. No. 09/931,948 to Lu et al., whichis incorporated by reference in its entirety.

FIG. 2 shows an inverted OLED 200. The device includes a substrate 210,an cathode 215, an emissive layer 220, a hole transport layer 225, andan anode 230. Device 200 may be fabricated by depositing the layersdescribed, in order. Because the most common OLED configuration has acathode disposed over the anode, and device 200 has cathode 215 disposedunder anode 230, device 200 may be referred to as an “inverted” OLED.Materials similar to those described with respect to device 100 may beused in the corresponding layers of device 200. FIG. 2 provides oneexample of how some layers may be omitted from the structure of device100.

The simple layered structure illustrated in FIGS. 1 and 2 is provided byway of non-limiting example, and it is understood that embodiments ofthe invention may be used in connection with a wide variety of otherstructures. The specific materials and structures described areexemplary in nature, and other materials and structures may be used.Functional OLEDs may be achieved by combining the various layersdescribed in different ways, or layers may be omitted entirely, based ondesign, performance, and cost factors. Other layers not specificallydescribed may also be included. Materials other than those specificallydescribed may be used. Although many of the examples provided hereindescribe various layers as comprising a single material, it isunderstood that combinations of materials, such as a mixture of host anddopant, or more generally a mixture, may be used. Also, the layers mayhave various sublayers. The names given to the various layers herein arenot intended to be strictly limiting. For example, in device 200, holetransport layer 225 transports holes and injects holes into emissivelayer 220, and may be described as a hole transport layer or a holeinjection layer. In one embodiment, an OLED may be described as havingan “organic layer” disposed between a cathode and an anode. This organiclayer may comprise a single layer, or may further comprise multiplelayers of different organic materials as described, for example, withrespect to FIGS. 1 and 2.

Structures and materials not specifically described may also be used,such as OLEDs comprising polymeric materials (PLEDs), such as disclosedin U.S. Pat. No. 5,247,190, Friend et al., which is incorporated byreference in its entirety. By way of further example, OLEDs having asingle organic layer may be used. OLEDs may be stacked, for example asdescribed in U.S. Pat. No. 5,707,745 to Forrest et al, which isincorporated by reference in its entirety. The OLED structure maydeviate from the simple layered structure illustrated in FIGS. 1 and 2.For example, the substrate may include an angled reflective surface toimprove out-coupling, such as a mesa structure as described in U.S. Pat.No. 6,091,195 to Forrest et al., and/or a pit structure as described inU.S. Pat. No. 5,834,893 to Bulovic et al., which are incorporated byreference in their entireties.

Unless otherwise specified, any of the layers of the various embodimentsmay be deposited by any suitable method. For the organic layers,preferred methods include thermal evaporation, ink-jet, such asdescribed in U.S. Pat. Nos. 6,013,982 and 6,087,196, which areincorporated by reference in their entireties, organic vapor phasedeposition (OVPD), such as described in U.S. Pat. No. 6,337,102 toForrest et al., which is incorporated by reference in its entirety, anddeposition by organic vapor jet printing (OVJP), such as described inU.S. patent application Ser. No. 10/233,470, which is incorporated byreference in its entirety. Other suitable deposition methods includespin coating and other solution based processes. Solution basedprocesses are preferably carried out in nitrogen or an inert atmosphere.For the other layers, preferred methods include thermal evaporation.Preferred patterning methods include deposition through a mask, coldwelding such as described in U.S. Pat. Nos. 6,294,398 and 6,468,819,which are incorporated by reference in their entireties, and patterningassociated with some of the deposition methods such as ink-jet and OVJD.Other methods may also be used. The materials to be deposited may bemodified to make them compatible with a particular deposition method.For example, substituents such as alkyl and aryl groups, branched orunbranched, and preferably containing at least 3 carbons, may be used insmall molecules to enhance their ability to undergo solution processing.Substituents having 20 carbons or more may be used, and 3-20 carbons isa preferred range. Materials with asymmetric structures may have bettersolution processability than those having symmetric structures, becauseasymmetric materials may have a lower tendency to recrystallize.Dendrimer substituents may be used to enhance the ability of smallmolecules to undergo solution processing.

The molecules disclosed herein may be substituted in a number ofdifferent ways without departing from the scope of the invention. Forexample, substituents may be added to a compound having three bidentateligands, such that after the substituents are added, one or more of thebidentate ligands are linked together to form, for example, atetradentate or hexadentate ligand. Other such linkages may be formed.It is believed that this type of linking may increase stability relativeto a similar compound without linking, due to what is generallyunderstood in the art as a “chelating effect.”

Devices fabricated in accordance with embodiments of the invention maybe incorporated into a wide variety of consumer products, including flatpanel displays, computer monitors, televisions, billboards, lights forinterior or exterior illumination and/or signaling, heads up displays,fully transparent displays, flexible displays, laser printers,telephones, cell phones, personal digital assistants (PDAs), laptopcomputers, digital cameras, camcorders, viewfinders, micro-displays,vehicles, a large area wall, theater or stadium screen, or a sign.Various control mechanisms may be used to control devices fabricated inaccordance with the present invention, including passive matrix andactive matrix. Many of the devices are intended for use in a temperaturerange comfortable to humans, such as 18 degrees C. to 30 degrees C., andmore preferably at room temperature (20-25 degrees C.).

The materials and structures described herein may have applications indevices other than OLEDs. For example, other optoelectronic devices suchas organic solar cells and organic photodetectors may employ thematerials and structures. More generally, organic devices, such asorganic transistors, may employ the materials and structures.

As used herein, the term “carbene” refers to compounds having a divalentcarbon atom with only six electrons in its valence shell when notcoordinated to a metal. A useful exercise to determine whether a ligandincludes a carbene-metal bond is to mentally deconstruct the complex asa metal fragment and a ligand, and to then determine whether a carbonatom in the ligand that was previously bound to the metal is a neutraldivalent carbon atom in the deconstructed state. The resonance forms ofa preferred embodiment may be shown as:

This definition of carbene is not limited to metal-carbene complexessynthesized from carbenes, but is rather intended to address the orbitalstructure and electron distribution associated with the carbon atom thatis bound to the metal. The definition recognizes that the “carbene” maynot technically be divalent when bound to the metal, but it would bedivalent if it were detached from the metal. Although many suchcompounds are synthesized by first synthesizing a carbene and thenbinding it to a metal, the definition is intended to encompass compoundssynthesized by other methods that have a similar orbital structure andelectron configuration. Lowry & Richardson, Mechanism and Theory inOrganic Chemistry 256 (Harper & Row, 1976) defines “carbene” in a waythat is consistent with the way the term is used herein. Some referencesmay define “carbene” as a carbon ligand that forms a double bond to ametal. While this definition is not being used in the presentapplication, there may be some overlap between the two definitions. Avariety of representations are used to depict the bonding in suchcarbenes, including those in which a curved line is used to indicatepartial multiple bonding between the carbene carbon and the adjacentheteroatom(s).

As used herein, the term “redox active metal” refers to a metal in acomplex, i.e., a metal ion surrounded by ligands, that can undergoreduction or oxidation. For the metal to be “redox active,” oxidationand reduction of the complex occur principally at the metal center, andnot on the ligands complexed to the metal. As a result, the oxidationstate of the metal changes upon reduction or oxidation of the complex,such that the electronic density on the ligands remains substantiallyunchanged, and the ligands, typically, do not dissociate from thecomplex.

As used herein, the term “redox active reducible metal complex” refersto a metal complex, as described above, where the metal is capable ofoxidation and reduction in an electrochemical process. For example, fora metal complex of formula TiX₄, where X is anionic ligand, the Ti is inthe 4+ oxidation state, i.e., Ti(IV) or Ti⁴⁺. Upon reduction, anelectron is added to the complex, and the complex takes a negativecharge, i.e., TiX₄ ⁻, and the Ti is reduced to its 3+ oxidation state,i.e., Ti(III) or Ti³⁺. As discussed above, the electron added in thereduction process is primarily located on the Ti center of the reducedcomplex.

As used herein, the term “readily switch redox state” refers to a metalcomplex in a first redox state that can readily switch to a second,accessible redox state at a moderate potential. As will be understood bythose of skill in the art, the redox state of a complex is a function ofthe number of electrons in the complex, which changes when the complexis reduced or oxidized. For a neutral complex, oxidation or reduction ina series of single steps, adding (reduction) or removing (oxidation)electrons, takes the complex through a series of redox states. When aneutral complex is reduced, the complex gains a negative charge, whichincreases with each successive reduction. Oxidation of a reduced complexreverses the process. If the voltage required to remove or add electronsto a complex is moderate, the redox state can be switched readily. Ifthe energy required to shift between redox states is high, the statewill not be switched readily. In addition, a high kinetic barrier mayexist for a change in redox state. As a result, the voltage required forthe change in state must be accessible in the matrix or solventcontaining the complex, and the kinetic barrier to the redox reactionmust be a small for the complex to readily switch redox state. Inaddition, the kinetic barriers to electron transfer can be minimized bya careful selection of a metal that has a low reorganization barrier toelectron transfer. The kinetic barriers for reduction of metal complexesare best described by Marcus electron transfer theory.

As used herein, the term “resting state” refers to the neutral state ofa vacuum deposited device. More generally, the resting state is theredox state of a complex before any charge is injected. For solutionprocessed OLEDs, where the complex is initially in the form of a cation,the cation is the resting state, and the neutral form of the complexcarries the electron.

Preferably, an OLED in accordance with the invention has at least oneorganic layer comprising a reversibly reducible metal complex with aredox active metal. This complex may be used in the device at least asan ETL or as a host material. The preferred reversibly reducible metalcomplexes readily undergo a reversible change of oxidation state, and,thus, may be referred to as reversibly reducible metal complexes. Thatis, for example, for a reversibly reducible metal complex comprisingTi(IV) as the metal, the Ti(IV) may be reduced to Ti(III) in the complexwithout the dissociation of a ligand, and then oxidized to Ti(IV). Thisprocess allows for the transport of electrons through the material.

A preferred OLED of the invention comprises an anode; a cathode; and atleast one organic layer disposed between the anode and the cathode. Theorganic layer comprises a reversibly reducible metal complex of a redoxactive metal center and at least one ligand, where, following areduction of the complex that adds one extra electron to the complex,the extra electron is predominantly localized on the metal center of thereduced complex. That is, as will be recognized by those skilled in theart, substantially all of the charge of the electron will be on themetal rather than on the ligands. The location of the electron in such areduced complex may be determined with X-Ray Absorption Fine Structure(“XAFS”) techniques, but is best determined through theoreticalcalculations, such as density function theory calculation (DFT), orElectron Paramagnetic Resonance (“EPR”).

Preferably, the metal is a d(0) metal, such as Ti(IV). By using a d(0)metal with ligands having an inherently high π* energy level, the extraelectron of the reduced species is localized on the metal center. Thelocalization of the electron on the metal of a reduced metal complexprevents bond formation between two reduced complexes. It may alsoimprove the stability of the complex, as the electron charge is notdistributed on the ligands, where irreversible bond breakage could leadto decomposition. As discussed above, in all prior art electrontransport materials, the electron added during reduction ispredominantly in a ligand based orbital.

Useful ligands preferably contain two electron donors, such as carbenes,that form a coordinatively saturated metal center, and do not dissociatefrom the metal center upon reduction of the complex. Carbenes areparticularly preferred, as carbenes are less likely to dissociate fromthe metal complex upon reduction of the complex, due to the strength ofthe carbene-metal bond. Particularly useful ligands include, but are notlimited to:

where E¹ and E³ are either/or nitrogen, oxygen, or sulfur mono anionicdonors, E² is a C, N, P, Si, etc neutral donor,

The reversibly reducible metal complexes useful in the invention areneutral and substantially colorless in the resting state, preferablyhaving an extinction coefficient at wavelengths from about 400 nm toabout 750 nm in the resting state of no more than about 1000 M⁻¹·cm⁻¹,and, more preferably, no more than about 100 M⁻¹·cm⁻¹. Preferredreducible metal complexes in accordance with the invention are ofFormula I

and Formula II

where M is a redox active transition metal, preferably a redox active d0transition metal; E¹ and E³ are independently selected from nitrogen,oxygen, and sulfur mono-anionic donors; E² is a C, N, P, or Si neutraldonor; (X-Y) is selected from a photoactive ligand or an ancillaryligand; m is a value from 1 to the maximum number of ligands that may beattached to metal M; and m+n is the maximum number of ligands that maybe attached to metal M, such that the overall complex is neutral.

Particularly useful reversibly reducible metal complexes include, butare not limited to complexes of formula:

where E¹ and E³ are either/or nitrogen, oxygen, or sulfur mono anionicdonors, and E² is a C, N, P, Si, or similar neutral donor,

It is understood that the various embodiments described herein are byway of example only, and are not intended to limit the scope of theinvention. For example, many of the materials and structures describedherein may be substituted with other materials and structures withoutdeviating from the spirit of the invention. It is understood thatvarious theories as to why the invention works are not intended to belimiting. For example, theories relating to charge transfer are notintended to be limiting.

The term “halo” or “halogen” as used herein includes fluorine, chlorine,bromine and iodine.

The term “alkyl” as used herein contemplates both straight and branchedchain alkyl radicals. Preferred alkyl groups are those containing fromone to fifteen carbon atoms and includes methyl, ethyl, propyl,isopropyl, butyl, isobutyl, tert-butyl, and the like. Additionally, thealkyl group may be optionally substituted with one or more substituentsselected from halo, CN, CO₂R, C(O)R, NR₂, cyclic-amino, NO₂, and OR,wherein each R is independently selected from H, alkyl alkenyl, alkynyl,aralkyl, aryl and heteroaryl.

The term “cycloalkyl” as used herein contemplates cyclic alkyl radicals.Preferred cycloalkyl groups are those containing 3 to 7 carbon atoms andincludes cyclopropyl, cyclopentyl, cyclohexyl, and the like.Additionally, the cycloalkyl group may be optionally substituted withone or more substituents selected from halo, CN, CO₂R, C(O)R, NR₂,cyclic-amino, NO₂, and OR.

The term “alkenyl” as used herein contemplates both straight andbranched chain alkene radicals. Preferred alkenyl groups are thosecontaining two to fifteen carbon atoms. Additionally, the alkenyl groupmay be optionally substituted with one or more substituents selectedfrom halo, CN, CO₂R, C(O)R, NR₂, cyclic-amino, NO₂, and OR.

The term “alkynyl” as used herein contemplates both straight andbranched chain alkyne radicals. Preferred alkyl groups are thosecontaining two to fifteen carbon atoms. Additionally, the alkynyl groupmay be optionally substituted with one or more substituents selectedfrom halo, CN, CO₂R, C(O)R, NR₂, cyclic-amino, NO₂, and OR.

The terms “aralkyl” as used herein contemplates an alkyl group that hasas a substituent an aromatic group. Additionally, the aralkyl group maybe optionally substituted on the aryl with one or more substituentsselected from halo, CN, CO₂R, C(O)R, NR₂, cyclic-amino, NO₂, and OR.

The term “heterocyclic group” as used herein contemplates non-aromaticcyclic radicals. Preferred heterocyclic groups are those containing 3 or7 ring atoms which includes at least one hetero atom, and includescyclic amines such as morpholino, piperidino, pyrrolidino, and the like,and cyclic ethers, such as tetrahydrofuran, tetrahydropyran, and thelike. Additionally, the heterocyclic group may be optionally substitutedwith one or more substituents selected from halo, CN, CO₂R, C(O)R, NR₂,cyclic-amino, NO₂, and OR,

The term “aryl” or “aromatic group” as used herein contemplatessingle-ring groups and polycyclic ring systems. The polycyclic rings mayhave two or more rings in which two carbons are common by two adjoiningrings (the rings are “fused”) wherein at least one of the rings isaromatic, e.g., the other rings can be cycloalkyls, cycloalkenyls, aryl,heterocycles and/or heteroaryls. Additionally, the aryl group may beoptionally substituted with one or more substituents selected from halo,CN, CO₂R, C(O)R, NR₂, cyclic-amino, NO₂, and OR,

The term “heteroaryl” as used herein contemplates single-ringhetero-aromatic groups that may include from one to three heteroatoms,for example, pyrrole, furan, thiophene, imidazole, oxazole, thiazole,triazole, pyrazole, pyridine, pyrazine and pyrimidine, and the like. Theterm heteroaryl also includes polycyclic hetero-aromatic systems havingtwo or more rings in which two atoms are common to two adjoining rings(the rings are “fused”) wherein at least one of the rings is aheteroaryl, e.g., the other rings can be cycloalkyls, cycloalkenyls,aryl, heterocycles and/or heteroaryls. Additionally, the heteroarylgroup may be optionally substituted with one or more substituentsselected from halo, CN, CO₂R, C(O)R, NR₂, cyclic-amino, NO₂, and OR.

MATERIAL DEFINITIONS

As used herein, abbreviations refer to materials as follows:

-   CBP: 4,4′-N,N-dicarbazole-biphenyl-   m-MTDATA 4,4′,4″-tris(3-methylphenylphenlyamino)triphenylamine-   Alq₃: 8-tris-hydroxyquinoline aluminum-   Bphen: 4,7-diphenyl-1,10-phenanthroline-   n-BPhen: n-doped BPhen (doped with lithium)-   F₄-TCNQ: tetrafluoro-tetracyano-quinodimethane-   p-MTDATA: p-doped m-MTDATA (doped with F₄-TCNQ)-   Ir(ppy)₃: tris(2-phenylpyridine)-iridium-   Ir(ppz)₃: tris(1-phenylpyrazoloto,N,C(2′)iridium(III)-   BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline-   TAZ: 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole-   CuPc: copper phthalocyanine.-   ITO: indium tin oxide-   NPD: N,N′-diphenyl-N—N′-di(1-naphthyl)-benzidine-   TPD: N,N′-diphenyl-N—N′-di(3-toly)-benzidine-   BAlq:    aluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate-   mCP: 1,3-N,N-dicarbazole-benzene-   DCM: 4-(dicyanoethylene)-6-(4-dimethylaminostyryl-2-methyl)-4H-pyran-   DMQA: N,N′-dimethylquinacridone-   PEDOT:PSS: an aqueous dispersion of poly(3,4-ethylenedioxythiophene)    with polystyrenesulfonate (PSS)-   HPT: 2,3,6,7,10,11-hexaphenyltriphenylene

While the present invention is described with respect to particularexamples and preferred embodiments, it is understood that the presentinvention is not limited to these examples and embodiments. The presentinvention as claimed therefore includes variations from the particularexamples and preferred embodiments described herein, as will be apparentto one of skill in the art.

1-13. (canceled)
 14. A reversibly reducible metal complex, selected fromthe group consisting of complexes of formula:

where E¹ and E³ are independently nitrogen, oxygen, or sulfur monoanionic donors, E² is a C, N, P, or Si neutral donor, wherein R1, R2,R3, R4, R5 and R6 are independently hydrogen or hydrocarbon groups,

wherein Ar is an aryl group.
 15. A host or electron transportingmaterial, comprising the reversibly reducible metal complex according toclaim
 14. 16. The host or electron transporting material according toclaim 15, wherein the metal complex is


17. The complex according to claim 14, wherein the complex is